I never let my dogs run with dogs I don't know, I just fail to see any reason to do so.
28347239874 miles of walks a day and 3298743874 hours of training, and my dog will be jumping all over the place. An hour of solid running and jumping and playing with other dogs, and she'll (mostly) settle down for the evening.
I know my dog, I keep a vigilant eye on other people/dogs, and the core group of dog owners who are at the park every day are pretty dog savvy and do a good job maintaining order. On rare occasion, I'll see a dog that makes me slightly nervous, and at that point I'll leash up Lucy and we'll go. I've had her 2 1/2 years, and never had an incident.